The present invention relates to a present position locating method.
The global positioning system (GPS) is widely known as a satellite positioning system, and is utilized for a car navigation system and the like. In the GPS, GPS satellite signals are respectively transmitted from a plurality of GPS satellites which orbit the earth, and a GPS receiver calculates (locates) its present position based on the received GPS satellite signals.
A GPS satellite signal affected by a multipath or the like may be included in the acquired GPS satellite signals. The term “multipath” refers to a phenomenon in which an indirect wave reflected or diffracted by a building or topography is superimposed on a direct wave from a GPS satellite so that the GPS receiver receives identical radio waves through multiple paths. Such a reception environment is referred to as a multipath environment. The present position of the GPS receiver may not be accurately calculated (located) when using a GPS satellite signal affected by a multipath. Specifically, it is necessary to perform positioning calculations while excluding a GPS satellite signal affected by a multipath or the like from the acquired GPS satellite signals. As a method of determining a GPS satellite signal affected by a multipath or the like, a method using an a priori residual (APR) has been known (see JP-A-2003-240836, for example).
The GPS receiver generally calculates its position as follows. Specifically, the GPS receiver selects satellite sets (i.e., combinations of four or more GPS satellites) based on the acquired GPS satellite signals, and performs positioning calculations corresponding to each satellite set to calculate a present position candidate. The GPS receiver selects the present position candidate considered to have the highest accuracy from the present position candidates calculated corresponding to the respective satellite sets based on an index such as a position dilution of precision (PDOP), and determines the selected present position to be the present positioning result. The PDOP is disclosed in JP-A-2006-058200, for example.
However, a plurality of GPS satellite signals may be affected by a multipath in a reception environment (e.g., multipath environment) with poor positioning accuracy. In this case, a GPS satellite with poor accuracy may be included in each satellite set. It is insufficient to merely determine a GPS satellite signal with poor accuracy. Specifically, it is necessary to appropriately determine and select a satellite set which is considered to have higher positioning accuracy.